Attachment 4
The Chemistry
of Polyurethane Coatings
Mobay Corporation
' USA INC COMPANY
Bayer
A General Reference Manual
Mobay Raw Materials for Urethane Coatings
Contents
I.
Introduction
II.
General Information
III.
Nomenclature
Monomeric Diisocyanates
• Toluene Diisocyanate (TDI)
• Hexamethylene Diisocyanate (HDI)
• Diphenylmethane 4,4-Diisocyanate ( MDI)
_____• Bis(4-Isocyanatocyclohexyl) Methane______
IV.
Polyisocyanates
• TDI Based Polyisocyanates
• HDI Based Polyisocyanates
• MDI Based Polyisocyanates
• IPDI Based Polyisocyanates
• Moisture-curing Polyisocyanates
_____* Blocked Polyisocyanates____________________
V.
VI.
Polyol Reaction Partners
• Desmophen and Multron Polyols
• Other Reaction Partners
10
Two-Component Coating Systems
• Polyisocyanate/Polyol Proportion • Levelling Agents
• Thickening Agents
• Properties
• Air Release Agents
• Modifiers
• Catalysts
• Solvents
• Curing
• Pigments and Extenders
___
_____• Matting Agents__________* Application___________
VII.
14
Moisture-Curing One-Component Coatings
• Prepolymers from Polyisocyanates and Polyols
• Desmodur E Polyisocyanates
• Properties
• Pigmented Coatings
• Curing
___
_____* Application___________________________________________________
VIII.
IX.___Repeatability__________________________1
5
X.
Storage_____________________________15
XI.
Health and Safety Information________________15
r
n
I. Introduction
During the late 1930s, Otto Bayer and coworkers pioneered the chemistry of polyisocyanates, a technology which led to the
advent of polyurethanes for a variety of ap­
plications. The principle reaction, shown
in Figure 1, occurs between an isocyanate
and an alcohol to form a urethane. The
reaction of difunctional isocyanates with
difunctional alcohols leads to polyure­
thanes. However, formation of films with
superior resistance to abrasion, chemicals,
and temperature extremes requires a threedimensional, crosslinked structure. This
can be readily accomplished with urethane
technology by employing at least one reac­
tion partner that contains three or more
reactive groups in the molecule. In many
applications, both the isocyanate and
alcohol reactants in two-component
systems are resins that contain multiple
functional groups.
Since 1955, Mobay Corporation has intro­
duced a variety of monomeric and poly­
meric isocyanates, polyesters, polyethers,
and acrylics for use in the formulation
of polyurethane coatings. Unlike alkyd
or melamine systems, polyurethane coat­
ings are based on well-defined stoichiometry. This feature allows the design of
urethane systems to meet specific end-use
requirements.
This brochure is intended to provide the
coatings formulator with background in­
formation on the chemistry of one- and
two-component polyurethane coating
systems. Further information from Mobay
Corporation in the form of Product Infor­
mation Bulletins and applications liter­
ature is available through your Mobay
representative.
O
R-NCO +
II
R'-OH ——-R-N-C-O-R'
1
H
Alcohol
O
II
R-NCO + R'-NH2 ——^R-N-C-NR'
1
1
H
H
Amine
use-
Urea
O
II
R-NCO + H2O —— [R-N-C-O-H] ——-R-NH2 + CO2
1
H
Carbamic Acid
Amine
Figure 1: Reactions of the isocyanate group
nOCN-R-NCO + nHO-R'-OH-
OCN-(R-N-C-O-R')-OH
I
n
H
Figure 2: Formation of urethane polymer
o
R
It
iMOVJ
C
R
N
N
1
1
p
O
C
// \.
O
N
1
/
^
O
R
II. General Information
The isocyanate group can react with any
compound containing a reactive hydrogen.
The three reactions shown in Figure 1 are
of principle interest. Reaction of an iso­
cyanate with an alcohol yields a urethane;
reaction of an isocyanate with an amine
yields a urea; and reaction of an isocyanate
with water will result in a carbamic acid
which is unstable and decomposes to yield
carbon dioxide and amine. Other poten­
tial isocyanate co-reactants include carboxylic acids, urethanes and ureas.
Urethane
F/gi/re 3: Formation of the isocyanurate ring
In order to prepare polymeric materials,
the reaction partners must each have at
least two functional groups per molecule
(Figure 2). Linear polymers are formed
when the reaction partners are each
difunctional. Three dimensional networks
require that at least one of the reaction part­
ners has three or more reactive groups.
The isocyanate group can also be made to
undergo self-condensation. For example,
three isocyanates can be trimerized to form
the isocyanurate ring as is shown in Fig­
ure 3.
III. Nomenclature
The trade names for the products dis­
cussed in this brochure are as follows:
• Desmodur and Mondur—Aliphatic
and aromatic diisocyanate
monomers and polyisocyanate
oligomers.
• Desmophen and Multron— Hydroxy
functional polyester, acrylic and
polyether reaction partners for
Desmodur and Mondur
polyisocyanates.
• Desmodur BL and Desmotherm—
Blocked polyisocyanates for baking
enamels.
• Desmocap—Blocked polyiso­
cyanates for room temperature cure.
• Crelan—Polyester and blocked
polyisocyanate raw materials for
thermoset powder coatings.
• Desmolac—High molecular weight
polyurethane lacquer resins.
• Baybond—Aqueous polyurethane
dispersions.
Monomeric diisocyanates are major build­
ing blocks for the value added products
most commonly used in the coatings in­
dustry. Mobay's polyisocyanate coatings
products are named with nomenclature
which stipulates product family, monomeric diisocyanate starting material and
weight solids. Familiarity with Mobay's
nomenclature will allow a quick determi­
nation of the chemical basis for most of
Mobay's coatings products. This nomen­
clature can best be understood by the
examples in Tables I and II. For
Desmodur E-21, the solids level of 100 %
is implied. This product designation could
also be written as Desmodur E-2100.
As with any system, notable exceptions ex­
ist and these relate to the product families
of Desmodur N and Mondur CB polyiso­
cyanates (Table HI). These long estab­
lished products predated the attempts
toward a systematic nomenclature. Addi­
tional details on the nomenclature are con­
tained in the following pages.
Desmodur E-21
L
% Solids———————
# in the series ————
Monomer Designation*Product Family**———
Trademark ——————
Desmodur N-3390
'Monomer Designation
1 = Toluene Diisocyanate—TDI
2 = Diphenylmethane—
4,4'-Diisocyanate—MDI
3 = Hexamethylene Diisocyanate—HDI
4 = Isophorone Diisocyanate—IPDI
5 = Bis(4-lsocyanatocyclohexyl)
Methane—H12MDI
"Product Family
BL = Blocked
E = Moisture Cure
N = Hexamethylene Diisocyanate
Based
Z = Isophorone Diisocyanate
Based
Table I: Nomenclature
Example
Desmodur E-1361
Desmodur E-21
Desmodur N-3390
Desmodur Z-4370
Family
Moisture Cure
Moisture Cure
HDI based
IPDI based
Monomer
% Solids
TDI
MDI
HDI
IPDI
61
100
90
70
Table II: Nomenclature examples
Polyisocyanates based on HDI:
Desmodur N-75
Desmodur N-100
Desmodur N-751
Polyisocyanates based on TDI:
Mondur CB-60
Mondur CB-75
Mondur CB-601
Mondur CB-701
Table III: Nomenclature exceptions
r
CH3
Jk/ NCO
OCN^
Cx
1
CH3
I /NCO
(Tj
NCO
2,4 isomer
2,6 isomer
Figure 4: Toluene Diisocyanate (TDI)
OCN
Figure 5: Diphenylmethane 4,4*Diisocyanate (MDI)
r
OCN-(CH2)6 -NCO
Figure 6: Hexamethylene Diisocyanate (HDI)
OCN
— ChU—<
)— NCO
Figure 7: Bis(4-lsocyanatocyclohexyl) Methane
IV. Monomeric Diisocyanates
r
There are important differences between
aromatic diisocyanate monomers and
aliphatic diisocyanate monomers. The
aromatic isocyanate groups are con­
siderably more reactive than the aliphatic
isocyanate groups, resulting in coatings
that dry faster and develop cure properties
faster than comparable systems based on
aliphatic isocyanates. Urethane products
made from aromatic diisocyanate mono­
mers oxidize more easily than do those
prepared from aliphatic diisocyanate mon­
omers, especially when exposed to UV
light. The higher resistance of products
prepared from aliphatic diisocyanates to
UV light-induced degradation means that
coatings based on them have better yellow­
ing and chalk resistance than those based
on aromatic diisocyanates. The products
based on aliphatic diisocyanates are there­
fore preferred for exterior topcoat applica­
tions which require color and gloss
retention.
Toluene Diisocyanate (TDI):
One of the most important monomers used
in the polyurethane industry today is
toluene diisocyanate (TDI). In the coatings
industry it is used mainly as a raw material
in preparing adducts and prepolymers
(Section V). TDI is also used as a modifier
of alkyd resins. The current method for in­
dustrial preparation of TDI produces two
isomers, shown in Figure 4. Mobay mar­
kets three versions of TDI under the fol­
lowing trademarks:
Mondur TD — 65/35 mixture of 2,4
and 2,6 isomers
Mondur TD-80 — 80/20 mixture of 2,4
and 2,6 isomers
Mondur TDS — 2,4 isomer
Mondur TD-80 is supplied in two different
grades varying in acidity.
Diphenylmethane 4,4-Diisocyanate
(MDI):
One of the first diisocyanate monomers
to achieve widespread industrial use is
diphenylmethane 4,4-diisocyanate
(MDI), shown in Figure 5. Mobay mar­
kets MDI under the trademark Mondur M
and supplies the material as a fused solid,
a flaked solid, or a liquid.
Hexamethylene Diisocyanate
(HDI):
Another important diisocyanate monomer
used for the preparation of a wide variety
of isocyanate adducts is hexamethylene
diisocyanate (HDI), shown in Figure 6.
This material is marketed by Mobay under
the trademark Desmodur H.
Bis(4-lsocyanatocyclohexyl)
Methane:
The monomeric diisocyanate shown in
Figure 7, bis(4-Isocyanatocyclohexyl)
methane, is marketed by Mobay under the
trademark Desmodur W.
Desmodur W is also known as hydrogenatedMDI (HMDIorH12 MDI), reduced
MDI (RMDI), or saturated MDI (SMDI).
It is a useful material for the production
of hydrolytically stable polyurethanes
as well as prepolymers for one- or twocomponent formulations, and for cast
elastomers.
V. Polyisocyanates
Industrial hygiene concerns limit the use
of monomeric diisocyanates directly in
coating applications. Higher molecular
weight species, such as adducts, prepolymers and isocyanurate trimers, are pre­
ferred because they contain only very low
levels of monomeric diisocyanate. A low
level of monomeric diisocyanate reduces
the concerns associated with manufactur­
ing and handling urethane coatings.
TDI Based Polyisocyanates:
As mentioned in Section IV, TDI has
gained particular significance in polyurethane coatings technology. Two different
TDI based polyisocyanate products are
marketed by Mobay, a TDI-trimethylolpropane (TMP) adduct and an isocyanur­
ate trimer. The idealized structures are
shown in Figures 8 and 9.
The various supply forms of the adduct
are marketed by Mobay under the trade­
mark Mondur CB. The trimer is mar­
keted by Mobay under the trademark
Desmodur IL.
Both Mondur CB and Desmodur IL can
be used in two-component coatings using
polyols as coreactants. Desmodur IL is
particularly suitable in applications where
extremely rapid curing two-component
poly urethane coatings are required.
O H
X NCO
II
1
CH2 -O-C-N — ^Aou
w CHa
/
OH
X NCO
/
II 1
CH3 -CH2 -C-CH2 -O-C-N — 0~CH3
\
OH
X NCO
\
II
1
CH2 -O-C-N — Q-CH3
Figure 8: Adduct of TDI and TMP
CH3
OCN
N
\ N /^\ NCO
CH3
Figure 9: Idealized structure ofDesmodur /L polyisocyanate
f
0 H
II
1
C-N-(CH2)6 - NCO
OCN-(CH2)6 -N
C-N-(CH2)6 - NCO
O
H
Figure 10: ThebiuretofHDI
NCO
1
(CH2)6
1
OCN-(CH2)6
^^ ^^
^^
^^ ^^
^^
1
XN
1
N
^C' X (CH2)6 -NCO
O
Figure 11: The isocyanurate ring of HDI
CH3
^^^
[1]
OCN
O
II
C
O
II
C
(CH2)6
N
1
^
/c
cx
O* X N X ^O
(CH2)6 -NCO
X N7 X N X
1
1
^c
c^
O* X N X X O
1
NCO
CH3
Figure 12: Copolymer of TDI and HDI
1 H
NCO
3
HDI Based Polyisocyanates:
Polyisocyanates based on HDI represent
the most important class of polyisocyanates used today in polyurethane coatings.
Coatings prepared with these products
have excellent resistance to chemicals and
abrasion and superior weathering char­
acteristics, including retention of gloss and
resistance to yellowing and chalking.
One commercial class of products in­
cludes Mobay's Desmodur N-75,
Desmodur N-100 and Desmodur N-3200,
which are polymeric materials that contain
biuret groups. An idealized structure of an
HDI-based biuret is shown in Figure 10.
Desmodur N-75 is a 75 % solids version of
Desmodur N-100. Desmodur N-3200 is a
low viscosity version of Desmodur N-100.
Like TDI, HDI can be converted to a
trimer that contains an isocyanurate ring
(Figure 11). These products, marketed un­
der the trademarks Desmodur N-3300 and
Desmodur N-3390, have better thermal
stability and weathering properties than
the biuret products. Desmodur N-3300
has a low viscosity similar to that of
Desmodur N-3200.
An isocyanurate copolymer of TDI and
HDI (Figure 12) is marketed under the
trademark Desmodur HL. As expected,
the properties of films prepared from
Desmodur HL are intermediate between
those prepared from the products
Desmodur N-3300 and Desmodur IL.
MDI Based Polyisocyanates:
In addition to the three forms of Mondur M
(monomeric MDI) discussed in Section
IV, oligomeric mixtures of the type shown
in Figure B, are marketed under the
trademarks Mondur MR, Mondur MRS,
Mondur MRS 4, and Mondur MRS 5.
These materials, supplied as low viscosity
liquids containing no solvent, are used to
prepare high solids or solvent-free coat­
ings, caulkings, and sealants. Either oneor two-component coatings can be pre­
pared from these polyisocyanates.
Isophorone Diisocyanate (IPDI)
Based Polyisocyanates:
Isocyanurate trimers based on isophorone
diisocyanate (Figure 14) are marketed
under the trademarks Desmodur Z-4370
and Desmodur Z-4570/2. These resins are
less reactive than Desmodur N polyiso­
cyanates and produce less flexible films.
However, Desmodur Z polyisocyanates
are compatible with aliphatic hydrocarbon
solvent blends and find use as additives in
alkyd systems.
Moisture-Curing Polyisocyanates:
In addition to the many polyisocyanates
that may be used in one-component appli­
cations, Desmodur E polyisocyanates
have been specifically developed for use
as moisture-curing systems. The reaction
of isocyanate groups with atmospheric
moisture forms the basis of this technol­
ogy. These products are supplied as higher
molecular weight polyurethane prepolymers containing reactive isocyanate
groups (Figure 15). For a variety of appli­
cations, Desmodur E polyisocyanates can
be used in the form supplied or as diluted
solutions. In addition to moisture-cure
systems, these products can be used in
two-component formulations.
4
NCO
NCO
NCO
n = 0 to 4
Figure 13: MDI Oligomers
"cO NCO
H3CLJ
HgC
C H2 -NCO
Figure 14: Isophorone Diisocyanate (IPDI)
R-NCO
R-NH2
+
+
H2O
R-NCO
—*
—
R-NH2 + CO2 t
R-NH-CO-NH-R
2 R-NCO
+
H2O
—
R-NH-CO-NH-R + CO2 |
Figure 15: Polyurea Formation
t
M
1
1
II
"
^
R-N /U ^BL
A
»^
D
n
R' - OH
Blocked Isocyanate
o
"' ^^c \ OR' -1- Rl H
M
INI
\Jr\
T DLn
BLH = phenol,
£-caprolactam,
butanone oxime
Figure 16: Cross/inking of Blocked Polyisocyanates
Blocked Polyisocyanates:
The polyisocyanates described thus far
contain unreacted isocyanate groups. In
typical two-component formulations,
these products are usually combined with
the polyol just prior to application. When
the isocyanate and polyol are mixed, they
immediately begin to react with one
another which leads to a limited, although
usually adequate, potlife. Another group
of Desmodur products contains isocyanate
groups that are blocked by compounds
such as £-caprolactam, butanone oxime
or phenol. At room temperature, these
blocked isocyanates do not react with
polyols at any appreciable rate. At elevated
temperatures, however, the blocked iso­
cyanate reacts with the polyol liberating the
blocking agent (Figure 16), which is
volatile and leaves the coating. This means
that one-component, room-temperature
stable coatings can be formulated with
blocked isocyanates and suitable polyol
components. Crosslinked polyurethane
films can be prepared at temperatures as
low as 230°F with properties similar to
those of corresponding two-component
systems.
A special group of blocked isocyanate
products are marketed under the
Desmocap trademark. These products
have been developed to crosslink at room
temperature with aliphatic amines.
Mobay supplies a wide variety of block­
ed polyisocyanates under the following
trademarks:
• DesmodurBL-1185A —
£-caprolactam blocked TDI
prepolymer
• Desmodur BL-1260A —
£-caprolactam blocked TDI
prepolymer
• Desmodur BL-3175A — Butanone
oxime blocked HDI isocyanurate
• Desmodur AP Stabil — Phenol
blocked TDI adduct
• Desmodur CT Stabil — Phenol
blocked TDI isocyanurate
• Desmocap 11A — Substituted phenol
blocked TDI prepolymer
• Desmocap 12A — Substituted phenol
blocked TDI prepolymer
VI.Polyol Reaction Partners
Desmophen and Multron
Polyols:
A wide variety of polyesters, polyethers,
and acrylics containing hydroxyl groups
are marketed by Mobay under
the trademarks Desmophen (polyesters,
polyethers and acrylics) and Multron
(polyesters).
Desmophen polyesters are available in
grades ranging from highly-branched to
linear with either high or low hydroxyl
content. In general, the product numbers
corresponding to the various grades of
Desmophen polyesters are a clue to the
structure — the lower the number, the
higher the degree of branching. As the
number increases, the polyesters become
increasingly linear and have a lower
hydroxyl content.
Desmophen polyacry lie resins can be dif­
ferentiated from the polyesters by the "A"
designation before the grade number; for
example; Desmophen A160, Desmophen
A 365, and Desmophen A 450.
Desmophen polyethers are designated by
the letter "U" following the number; for
example; Desmophen 500U and
Desmpohen 1600U.
Other Reaction Partners:
Oils and alkyds can be used in combina­
tion with Mobay polyisocyanates. Forex­
ample, various short oil alkyds can be used
with Desmodur HL and Desmodur IL
polyisocyanates. Castor oil which contains
hydroxyl groups is a potential reaction
partner for Mondur CB and Mondur MR.
Mondur CB and Desmodur N combined
with epoxy resins containing secondary
hydroxyl groups give films with good
adhesion and resistance to aggressive
chemicals. Certain phenolic resins also
give particularly good water-resistant
films when they are combined with
Mondur CB polyisocyanates.
Silicone resins with hydroxyl groups also
react with polyisocyanates. For example,
Baysilone UD resin manufactured by
Bayer AG, West Germany, when crosslinked with Mondur CB polyisocyanate at
room temperature, produces coatings that
maintain their mechanical properties
longer, even after prolonged treatment of
the films at temperatures as high as 180 °C.
However, a certain degree of yellowing
will occur.
Vinyl polymers containing hydroxyl
groups are also suitable reaction partners
for Mobay polyisocyanates. Such systems
can be used for coating wood and plastic
substrates.
Other products which are suitable for use
with polyisocyanates include cellulose
esters, ketone resins, bituminous coal tar,
and bitumen.
VII. Two-Component Coating
Systems
The most frequently recommended prod­
ucts for two-component applications are
Mobay's Desmodur polyisocyanates and
Desmophen polyols (DD). In addition, for
some applications Mobay's Mondur poly­
isocyanates and Multron polyesters may
also be appropriate. The term DD coating
refers to any of the two-component coat­
ings based on polyisocyanates and polyols
available from Mobay.
Calculations:
When preparing two-component coating
formulations, the rules for chemical equa­
tions are followed. In theory, a maximum
molecular weight is reached and those
properties associated with molecular
weight are optimized when one equivalent
of isocyanate reacts with one equivalent of
hydroxyl, that is, when the ratio of NCO
toOHis l.Oto 1.0 (Equation 1, Figure 17).
In practice, a small excess of isocyanate,
about 10%, is often used to allow for the
destruction of isocyanate by the likely
presence of water in pigments or solvents,
so that the NCO to OH ratio of 1.0 to 1.0 is
maintained. In addition, it is sometimes
desirable to vary the NCO to OH ratio
from about 0.9 to 1.0 to about 1.2 to 1.0 in
order to modify the properties of the
coating. In any event, it is of great impor­
tance that the NCO/OH be controlled.
In order to achieve a ratio of NCO to OH
of 1.0 to 1.0, the weight of one equivalent
of the supplied form of the polyisocyanate
is reacted with one equivalent of the sup­
plied form of the polyol. Usually the prod­
uct literature will list the equivalent
weights, but if not, then the polyisocyanate
literature will give the % NCO and the
polyol literature will give the % OH or the
OH value. The NCO equivalent weight
can be calculated from the % NCO (Equa­
tion 2, Figure 17); and the OH equivalent
weight can be calculated from either the %
OH or the OH value (Equation 3, Figure
17). These equations can be combined to
calculate the weight ratios of ingredients
for any NCO/OH. Basic algebra is all that
is necessary for these basic manipulations.
Sample calculations are shown in Fig­
ure 18.
1. n(OCN— R— NCO) + n(HO— R'—OH) —+•
OH
HO
II I
I II
(--C— N— R— N— C— O— R'—O—)
Urethane Polymer
n
2. Equivalent weight (Isocyanate-containing resin) =. 42 x 100
%NCO
a (a) Equivalent weight (Hydroxyl-containing resin)
17 x 100
(b) Equivalent weight (Hydroxyl-containing resin)
56,100
%OH
OH Number
(OH Value)
Figure 17: Equations for polyisocyanate/polyol reaction
Determine the amount of Desmodur N-100 polyisocyanate required to react with 300 Ibs
of Multron R-221-75 polyol. Use a 1.2 to 1.0 NCO to OH ratio.
Desmodur N-100 contains 22% NCO
Multron R-221-75 contains 33% OH
a) Equivalent weight of N-100 =
40 )f -inn
22
-191
Equivalent weight of R-221-75 _ 17 x 10° _ 515
3.3
For the theoretical reaction, it is necessary to use 1 equivalent or 191 Ibs of N-100
with 1 equivalent or 515 Ibs of R-221-75.
b) The number of equivalents in 300 Ibs of R-221-75 - 300 - 058
515
c) At 1.0 to 1.0 NCO to OH ratio, equivalents of N-100 required are 0.58.
/. The amount of N-100 = 058 x 191 =111 Ibs
d) Since a reaction ratio of 1.2 to 1.0 of NCO to OH is desired, then the amount of N-100
required is 058 x 1.2 x 191 = 133 Ibs
Figure 18: Example Problems
10
Properties:
The properties of a polyisocyanate/polyol
coating depend on certain factors, such as
the degree of branching of the reaction
partners, the content of reactive groups
and the physical state of the individual raw
materials.
It is possible to change the film properties
by using the same poly isocyanate with dif­
ferent polyols while maintaining a con­
stant stoichiometric ratio. The higher the
hydroxyl content of the particular reaction
partner, the harder and die more chemical­
ly resistant the resultant films will general­
ly be. A low hydroxyl content normally
gives softer and more flexible films.
A further possibility for modifying the
film properties is to use the same polyol
and vary the quantity of the polyisocyanate. By "undercrosslinking," i.e.,
NCO:OH<1, the polyurethane films
generally become more flexible though
they are less weather-resistant and less
resistant to solvents and chemicals. On the
other hand, by exceeding the stoichiomet­
ric quantity of poly isocyanate, or "overcrosslinking," i .e., NCO: OH > 1, the resultant films tend to be harder and have
greater chemical resistance.
Modifiers:
In addition to the two main components,
DD coating systems frequently contain
other modifying constituents. These are
added to improve specific application
properties (levelling agents or thickeners
for example), and are normally incorpo­
rated in the order of 1-10% of the total
binder. For certain applications, it may be
best to formulate coatings containing more
than 35 % modifying binder constituents.
In addition to cellulose acetate butyrates,
low molecular weight acrylic resins and
polyvinyl chloride/polyvinyl acetate
(PVC/P\AC) copolymers are suitable
modifiers. Coatings prepared from com­
binations of nitrocellulose and aromatic
isocyanates tend to yellow substantially
and are predominantly used for primers
and as clear varnishes for dark substrates.
However, combinations with the aliphatic
Desmodur N polyisocyanates do not
yellow.
Solvents:
Suitable solvents for two-component systerns include esters, ketones, and ether
esters. Possible diluents are aromatic
hydrocarbons such as toluene, xylene or
higher-boiling aromatic petrochemical
hydrocarbons. Desmodur Z polyiso­
cyanates are compatible with aliphatic di­
luents. Chlorinated hydrocarbons may be
used only in systems that do not contain
finely divided metal additives. The choice
of solvents and diluents should be evalu­
ated for each coating system.
Should it be necessary to dilute polyiso­
cyanates, special care should be taken
when selecting the solvents. Any solvent
chosen must not contain hydroxyl groups.
Therefore, alcohols or any solvent con­
taminated with water should not be
used. It is not recommended to dilute
polyisocyanates below 35% solids
content since precipitation of resins may
occur.
The water content of the solvents or sol­
vent mixes for polyisocyanates should
not exceed 0.05 %. Urethane grade sol­
vents are suitable for polyols, as well as
polyisocyanates.
Solvents which contain reactive groups,
e.g., amines, should not be used since they
react with isocyanate groups.
Regulations and safe handling procedures
governing combustible liquids must be
observed.
Pigments and Extenders:
The following inorganic pigments are
highly suitable:
White: TiO2 types
Yellow: Iron oxide yellow types, nickel
and chrome titanates, light
yellow types, chrome yellow and
cadmium types
Brown: Iron oxide brown types
Red: Iron oxide red pigments,
cadmium types
Black: Iron/manganese mixed metal
oxide black, iron oxide black
types (not for grey shades) and
some carbon blacks
Blue: Light blue and
chrome oxide green types
A marked reduction in the potlife may
be expected by using the following pig­
ments: zinc oxide, red lead, lead cyanamid, molybdate red and some carbon
blacks. Zinc and lead chromates, as well
as zinc phosphate, have gained special
importance as passivating pigments for
corrosion protection in primers. However,
these pigments are only suitable if they do
not excessively shorten the potlife. Zinc
dust may be used for anti-corrosion
primers as well as non-leafing aluminum
for barrier type primers.
The following organic pigments are
suitable:
Blue: Phthalocyanine blue types
Green: Phthalocyanine green types
Red: Perylene types and quinacridone
types
Organic pigments may catalytically ac­
celerate the curing reaction. Their suit­
ability is best established by testing the
potlife. For organic pigments, normally
not more than approximately 6 %, calcu­
lated on solid binder, should be used for
glossy films. Note that organic pigments
will not always give sufficient coverage in
single-coat applications.
In addition to transparent pigments, solu­
ble dyestuffs can also be used to obtain
transparent shades. Soluble dyestuffs do
not generally have the same light fastness
as suitable organic pigments. Metal com­
plex dyestuffs can also be used.
Conventional extenders are barytes, heavy
spar, microtalc, kaolin, micaceous iron
oxide, magnesium mica, asbestos flour,
quartz flour, powdered slate, and silicon
carbide. It is advisable to carry out evalua­
tions prior to use.
Matting Agents:
The incorporation of conventional matting
agents based on silica allows any desired
level of gloss to be obtained, either with
clear or pigmented DD coating systems.
Incorporating polyolefin wax is also
advantageous. Micronized polypropylene
waxes can be incorporated without diffi­
culty and improve the appearance of the
matted surface. In the case of readily
dispersible products, it is sufficient to mix
them simply by high-speed stirring.
To obtain a matt or eggshell gloss effect,
4-15 % matting agent, calculated on binder,
is generally required. This quantity will
vary, depending on the composition of the
polyol solution or mill base.
The flattening effect can often be assisted
by the incorporation of components which
increase the viscosity and/or improve
physical drying.
11
Levelling Agents:
Apart from the choice of solvents, the
addition of suitable levelling agents can
improve the flow properties when
necessary.
Cellulose acetate butyrate or low
molecular weight acrylic resins, depend­
ing on compatibility, are used in quantities
of about 0.2-2%, calculated on solid
binder.
Nitrocellulose also has the effect of im­
proving the flow and promoting pigment
wetting. However, because of the pro­
nounced yellowing in connection with
aromatic polyisocyanates, nitrocellulose
can only be used with these materials in
applications where light stability is not
important.
Poly vinyl acetate, poly vinyl butyral, copolymers of PVC/PVAC, as well as certain
urea resins, may improve levelling proper­
ties. The added quantities are usually be­
tween 0.5% and 3%, calculated on the
binder. Silicone fluids or fluorochemical
surfactants lower the surface tension to im­
prove flow.
Thickening Agents:
Certain application methods, such as cur­
tain coating, require increased viscosity of
the DD coating. Suitable thickening agents
are copolymers of vinyl chloride and vinyl
acetate, polyvinyl butyral, nitrocellulose,
silicas, thickeners of bentonite and hydrogenated castor oil.
If an increase in the viscosity is desired,
copolymers of vinyl chloride and vinyl
acetate and polyvinyl butyral can be added
to the polyol solution in quantities of
5-10%, based on binder.
Nitrocellulose can be added in quantities
of 5-10%, based on polyisocyanate/polyol,
resulting in an appreciable increase in vis­
cosity. One disadvantage, however, can be
the pronounced yellowing effect when
combined with aromatic polyisocyanates.
Hydrogenated castor oil will produce thixotropy, which allows thick films to be ap­
plied even to vertical surfaces. Additions
of 1% based on resin solids are often suf­
ficient.
Silicas increase the viscosity and also pro­
duce thixotropy. An addition of 3 % is often
adequate and has only a minor influence
on the degree of gloss. These products are
best suspended with solvents in a dissolver
before use.
Bentonite thickeners are used in additions
ofupto 1.5% on solid binder to prevent the
settling of pigments and extenders. These
agents are also best suspended with
solvents before use.
When selecting thickeners, their compat­
ibility with polyols must be considered.
Furthermore, additions that impair the
characteristic properties of the coatings
must be avoided. This particularly applies
to coatings based on Desmodur N
polyisocyanates.
Air Release Agents:
Air release agents are particularly useful
for the prevention of blistering during ap­
plication by brush, curtain coating or roller
coating. Special copolymers of acrylic
esters (Modiflow, Monsanto) have proven
to be suitable. Although these products are
often incompatible with DD coating mate­
rials, the slight turbidity which can be
observed in the unpigmented coatings is
no longer noticeable in the dry film if the
correct amount is added. The addition
should be a maximum of 0.05 % based on
solid binder.
Catalysts:
Catalysts are used in DD coating systems
to shorten the curing time, especially in
systems containing aliphatic isocyanates.
They differ considerably in the extent to
which they accelerate the reaction.
Desmorapid PP, a tertiary amine, is often
a suitable catalyst. Various metal com­
pounds are also suitable. Dibutyltin
dilaurate or zinc octoate, for example, are
of particular significance in combination
with Desmodur N aliphatic polyisocyanate products or in moisture-curing
one component coatings. While accel­
erating the cure, the use of a catalyst will
also shorten the potlife.
The required addition level and effective­
ness of a catalyst will vary. An excessive
amount of catalyst may impair the devel­
opment of film properties such as hard­
ness, abrasion resistance, UV resistance
and appearance of the film. It is advisable
to carry out evaluations prior to making
a choice.
Curing:
Curing of the DD coating systems can be
carried out at room temperature. The dry­
ing times can vary considerably depending
on the types of polyisocyanate used. The
choice of the polyol to be reacted with a
polyisocyanate wUl also affect the system's
dry time. In general, systems based on the
following polyisocyanates dry at room
temperature in these approximate relative
times.
Desmodur IL polyisocyanate 1
Desmodur HL polyisocyanate 2
6
Mondur CB polyisocyanates
Desmodur N polyisocyanates,
6
with catalyst
Desmodur N polyisocyanates 30
I
The reaction between the poly isocyanate
and polyol can occur at temperatures as
low as 32 °F (0 °C) but, in practice, eleva­
ted temperatures are often used for forced
drying in production line coatings.
The highly reactive Desmodur IL and
Desmodur HL polyisocyanates have
particular importance for baked finishes
as they allow extremely short drying
times. Coatings based on Mondur CB or
Desmodur N polyisocyanates can also be
dried at elevated temperatures in order to
reduce the cure time.
Aliphatic polyisocyanates, such as the
Desmodur N polyisocyanates, have low
reactivity by nature, and longer drying
times can be expected. The incorpora­
tion of a catalyst such as 0.005 % dibutyltin
dilaurate, 0.2% zinc octoate or 0.5%
Desmorapid PP, on solid binder, reduces
the drying time to practical levels.
While Mondur CB poly isocyanate based
coatings normally do not require a cata­
lyst, the addition of an accelerator may
be advisable for specific applications.
Metal-based accelerators have less of an
effect on Mondur CB polyisocyanates than
the preferred amine based catalysts, e.g.,
Desmorapid PP. Depending on their ef­
fectiveness, additions range between ap­
proximately 0.05% and 0.2%, on solid
binder.
Coatings that are based on Desmodur IL
or Desmodur HL polyisocyanates are nor­
mally used without a reaction accelerator.
Application:
The chemical curing mechanism of DD
coatings makes it necessary to consider the
following points:
1. Both components must be thor­
oughly mixed together. In some
cases it is advantageous to allow the
mixture to stand for half an hour
before application.
2. The reaction which begins imme­
diately after mixing, results in a
gradual increase in viscosity.
3. The increase in viscosity even­
tually leads to gelation of the paint.
The useable potlife depends on the follow­
ing factors:
• The binder concentration.
• The NCO/OH ratio.
• The selected polyisocyanate.
• The selected polyol.
• The catalyst type and
concentration.
• The temperature of the coating
formulation.
• The choice and quality of the
solvents.
• The possible effect of any incor­
porated auxiliaries, pigments, or
extenders.
If these facts are taken into account and
the coating is properly formulated, the
potlife can normally be set to be one work­
ing day. In principle, this means that the
coating can be applied by any conventional
method.
DD coatings can be applied by a variety of
methods. Application by spray, brush,
conventional roller coating, and curtain
coating is possible. Since curtain coating
requires relatively high initial viscosity, it
is essential that the two-component com­
bination exhibits a slow increase in vis­
cosity at the proper consistency.
Brushable coatings can be formulated
by adjustment of viscosity and dry time.
Brushable coatings can generally also be
applied by airless spraying and in many
cases, by the hot-spray method at approx­
imately 110°F (80 °C). In general, applica­
tion by the electrostatic spray method is
possible due to the polarity ofthe coatings.
Spray coatings applied by the conventional
compressed air method require relative­
ly low viscosity and the use of a suitable
solvent mix that evaporates as rapidly
as possible. Two-component spray guns
which automatically meter and mix the
components are useful.
Gelation of the remaining portion of the
coating, e.g., with dip baths or curtain
coating machines, can be delayed by dilu­
tion with the amount ofpolyol required for
the next period of work. The required
quantity of polyisocyanate should not be
added until shortly before recommencing
the work. The effect can be made even
more pronounced by adding 10% solvent
to the paint mix. Mixing must be done
carefully and thoroughly.
A coating which has already been mixed
can be kept overnight or during long
breaks by chilling or refrigeration. This
method can be combined with the pro­
cedure described above.
r
13
VIII. Moisture-Curing
One-Component
Coatings
Soluble adducts of diisocyanates or
polyisocyanates and polyols with an excess
of isocyanate groups ("prepolymers") can
crosslink with atmospheric moisture
(Figure 19) to give insoluble higher mo­
lecular weight polyurethane/polyureas.
This reaction describes the curing princi­
ple for moisture-curing polyurethane
coatings.
There are two ways of formulating
one-component coatings of this kind:
a) Preparation of prepolymers
from suitable polyisocyanates and
polyols, or
b) Use of DesmodurE polyisocyanates.
Prepolymers from Polyisocyanates
and Polyols:
Mobay's Desmodur N, Mondur CB and
Mondur MRS polyisocyanates are suitable
for the preparation of prepolymers for the
formulation of one-component coatings.
The prepolymers are normally prepared
from polyols of low functionality and a
stoichiometric excess of isocyanate. A
moisture-curing, clear varnish can be
made as follows: The poly isocyanate and
solvent are mixed for a short time in the
dissolver under the exclusion of moisture.
If a moisture scavenger is required, para
toluene sulfony 1 isocyanate (pTSI) can be
added. After this, the polyol, other addi­
tives, and if necessary, accelerators, are
added and the mixture is packed in a dry,
airtight container. The coating reaches its
final viscosity after approximately seven
days. The storage stability of the final prod­
uct is dependent on the polyol and should
be closely examined.
Desmodur E Polyisocyanates:
The name Desmodur E covers a range of
ready-to-use, one-component coatings
which require no other treatment except
for thinning in some cases. Because they
are far simpler to use than the two-com­
ponent coatings, the Desmodur E polyiso­
cyanates have become very popular.
Properties:
The properties of moisture-curing onecomponent coatings are principally deter­
mined by the nature of the particular base
isocyanate. For example, one-component
coatings based on aliphatic isocyanates
generally need longer drying times than
those based on aromatic isocyanates. The
14
R-NCO
R-NH2
+
+
H2O
R-NCO
—
—*
R-NH2 + CO2 t
R-NH-CO-NH-R
2 R-NCO
+
H2O
—
R-NH-CO-NH-R + CO2 |
Figure 19: Polyurea Formation
drying times depend not only on the tem­
perature but also on the amount of atmos­
pheric moisture. With very low absolute
moisture content (e.g., in winter) the dry­
ing times may be increased.
The weathering properties of one-com­
ponent coatings also largely depend on the
type of isocyanate used. Types based on
TDI or MDI have a tendency to yellow in
the light and to show a relatively rapid loss
of gloss on weathering; types based on
HDI are light-stable. Depending on the
composition, HDI based coatings may
also be equivalent in gloss retention and
chalking behavior to two-component
coatings that are based on Desmodur N
polyisocyanates. One-component urethane coatings have very good mechanical
properties. The films can range from hard
to very flexible. Their surfaces are par­
ticularly mar-resistant, and their abrasion
resistance is exceptionally high. The films
also have good resistance to chemicals, in­
cluding the stronger organic acids as well
as alkalis, alcohols, solvents and water.
Pigmented Coatings:
Due to the sensitivity of one-component
coatings to moisture, a special technique
has to be followed when formulating pigmented coatings. This involves the use of
pTSI and Additive OF and splitting the
grind procedure into several steps. This
process may be accomplished in the same
amount of time as the milling of an alkyd
resin and can be done with the same
equipment.
The formulation is divided into four
operations:
1. Weighing
2. Predispersing
3. Dispersing in the sand mill
4. Filling
As explained in Figures 20 and 21, the in­
dividual components are weighed in the
given sequence and predispersed with the
dissolver. In so doing, two processes oc­
cur simultaneously:
a) Homogenization of the weighed
materials.
b) Dehydration of the pigments,
extenders and solvents by pTSI.
Carbon dioxide, released through the
chemical reaction between water and
pTSI, acts as a buffer gas and prevents
contact with air and humidity. While the
dissolver charge is warm, it can be dis­
persed in the sand mill.
After cooling, the catalyst, and if neces­
sary Additive OF, can be added and the
formulation adjusted to the desired
application viscosity with anhydrous
solvents. The material should then be
packed in dry, airtight containers.
Another method of formulating pigmented coatings is to add the pigment in
the form of a paste colorant. In this case,
the pigment mill base can be prepared in
a binder which reacts as little as possible
with the NCO group. The incorporation
of pTSI ensures that the pigment paste is
free of water. The paste may then be mixed
with the Desmodur E binder (such as
Desmodur E-21).
Curing:
The drying rate of one-component
coatings is dependent on the relative
atmospheric humidity and temperature.
Low temperature and low atmospheric
humidity may slow down the drying
considerably.
As with two-component systems, onecomponent coatings based on aliphatic
polyisocyanates often require the incor­
poration of reaction accelerators. Metal
compounds are especially suitable for this
purpose.
v>
Polyisocyanate
solvent
pTSI
other additives
pigments
extenders
1. Weighing
CO2
— — - — —
-_—_- — —
~— *^r j~ —
— — — —
2. Predispersing
— high viscosity—
and dehydration
up to 60 °C
(15 min.)
Elevated temperature
is important
Polyol addition
further 15 min.
(60°C)
This does not, of course, apply to such
systems designed for overcoating at a later
date, such as shop-primers or those spe­
cifically formulated for use as primers or
adhesion promoters.
c-^-.—
—— 1_-_
_
_
_ _ z
~^~^
n
~k.
4
X. Storage
~~\ ~ ~
3. Sand Mill
4. Filling into cans
Catalyst addition to
lightfast coatings
(0.05-0.1% dibutyltin
dilaurate), leave
to stand in a warm
place overnight
Viscosity becomes
stable after
a few days
Figure 20: Formulation ofpigmented one-component coatings from polyisocyanate/polyol
Desmodur E
solvent
pTSI
^- -^
,——— ———,
other additives
C<^2
pigments
extenders
~- ~— ~ |-~-~_- - — -_•
1. Weighing
2. Predispersing
—high viscosity—
and dehydration
up to 60 °C
(15 min.
Elevated temperature
is important
|V ,y
tr-^-zr!
~ ~ — ~t
~^r. _—_-
^^
"~~
^"^^*.
'^~^-:^>-
M
a Sand Mill
4. Filling into cans
Maturing time
approximately
16 hours
Catalyst addition to
lightfast coatings
(0.05-0.1% dibutyltin
dilaurate)
Figure 21: Formulation ofpigmented one-component coatings from
Desmodur E polyisocyanates
Application:
Moisture-curing coatings based on the
Desmodur E polyisocyanates are normally
applied by brushing or spraying. Dip coat­
ing and curtain coating cannot generally
be used because of the extended contact
between the liquid coating and atmos­
pheric moisture.
Unpigmented one-component coatings
are used principally for wood substrates
(parquet, indoor application). Another
area of application is the impregnation or
coating of concrete which also includes
decorative seamless flooring.
Pigmented one-component coatings can
be used, among other things, for anti-
corrosion coatings for metals, for the sur­
face treatment of concrete or asbestos ce­
ment and for various decorative coatings.
IX. Recoatability
Good intercoat adhesion is necessary for
multi-coat systems. Prolonged coating
intervals may impair intercoat adhesion
of one- and two-component coatings.
Coating intervals should not exceed 24 to
28 hours depending on the degree of
through hardening.
In the case of longer coating intervals,
slight intermediate sanding or appropri­
ate priming may be required to ensure
adhesion.
Particular care must be given to the storage
of any Mobay poly isocyanate that contains
isocyanate groups. Containers of these
products must be kept tightly sealed since
their reaction with atmospheric moisture
leads to an increase in viscosity and, even­
tually, gelation. Since carbon dioxide is a
product of the reaction of isocyanates with
water, pressure buildup and possible rup­
ture of sealed containers of contaminated
polyisocyanates can occur.
Containers used to store polyisocyanates
should be inspected prior to use to be cer­
tain they are clean and dry.
Moisture should also be excluded from
stored polyester, polyether, or acrylic
coreactants. Most of these materials are
hygroscopic and will absorb and retain
atmospheric moisture. Water contamina­
tion of the polyols may lead to inferior
film properties when combined with
polyisocyanates.
All legal regulations concerning the
storage of Mobay products must be
observed.
XI. Health and Safety
Information
Appropriate literature has been assembled
which provides information pertaining to
the health and safety concerns that must be
observed when handling Mobay products
mentioned in this publication. For mate­
rials mentioned that are not Mobay prod­
ucts, appropriate industrial hygiene and
other safety precautions recommended by
their manufacturer should be followed.
Before working with any product
mentioned in this publication, you must
read and become familiar with available
information concerning its hazards, prop­
er use, and handling. This cannot be over­
emphasized. Information is available in
several forms, e.g., material safety data
sheets and product labels. Consult your
Mobay representative or contact the In­
dustrial Hygiene and Regulatory Com­
pliance Group of the Coatings Division.
15